The present invention relates to a defect detection method and an apparatus therefor in which an image of a target such as a semiconductor wafer, a TFT, and a photo mask, which is obtained using light or a laser beam, is compared with a reference image stored beforehand, and then a minute pattern defect, a foreign material, and the like are inspected judging from a difference obtained by the comparison. In particular, the present invention relates to a defect detection method and an apparatus therefor that are suitable for visual inspection of a semiconductor wafer.
The prior art in which an image to be inspected is compared with a reference image so as to detect a defect is disclosed by Japanese Patent Laid-Open No. 05-264467.
This method comprises the following steps: successively picking up a sample to be inspected, on which repeated patterns are regularly arranged in rows, using a line sensor; comparing the picked-up image with an image delayed by a period of time equivalent to a repeated pattern pitch; and detecting disagreement between the images as a pattern defect. However, in reality, there are the vibration of a stage and the inclination of a target, which may cause the positions of two images to become misaligned. Accordingly, it is necessary to determine the quantity of displacement of the image picked up by the sensor relative to the image delayed by a period of time equivalent to a repeated pattern pitch. In addition, after aligning two images on the basis of the quantity of displacement which has been determined, a difference between the images is calculated. Then, if the difference is larger than a predetermined threshold value, it is judged to be defective; or if not, it is judged to be nondefective.
As described above, in the prior art, a defect is inspected by the steps as follows: detecting displacement of the image to be inspected relative to the reference image, aligning a position of the image, and then, comparing the images (that is to say, calculating a difference between the two images), followed by extracting a defect. However, if the quantity of displacement is not correctly determined when the displacement is detected, the alignment will be performed at a wrong position. In this case, on the image to be aligned, a difference of the image to be aligned does not become large in an area where a change in brightness is small; however, the difference also becomes large in an area where a change in brightness is large. For example, reference numeral 11 in FIG. 1A is an image to be inspected; reference numeral 12 is an example of a reference image; reference numeral 1a is a uniformly bright background area; and reference numeral 1b is an area having a dark pattern on a bright background. In addition to it, the image to be inspected 11 includes a defect 1c. A waveform of luminance values along line 1D–1D′ on the image of this example plots a graph as illustrated in FIG. 1B.
When the quantity of displacement of the image to be inspected 11 relative to the reference image 12 is correctly determined, a difference image as shown in FIG. 2 is produced after the alignment of the image to be inspected 11 with the reference image 12. The difference image is an image which is shaded according to a difference value between corresponding positions of the image to be inspected and the reference image. If a part, a difference value of which is larger than or equal to a specific threshold value TH, is regarded as a defect, only the defect 1c of the image to be inspected 11 is detected in FIG. 2. However, if the quantity of displacement of the image to be inspected 11 relative to the reference image 12 is incorrectly calculated, a difference image as shown in FIG. 3B is produced after the alignment. In an area like pattern edges in the area 1b where a change in luminance value is large, even slight displacement causes a difference value to become larger. Therefore, the area is detected as a defect. Basically, this should not be detected as a defect. In other words, it is a false report.
Conventionally, as a method for preventing a false report from occurring as shown in FIG. 3A, a threshold value TH is increased to TH2 as shown in FIG. 3B. This method decreases sensitivity, with the result that a defect having a difference value smaller than or equal to that of the edges cannot be detected. Alternatively, as shown in FIG. 3C, a high threshold value like TH2 is used in a high-contrast part where a false report easily occurs; and the threshold value TH which is lower than TH2 is used in a low-contrast part where a false report rarely occurs. This means that a plurality of threshold values has to be handled, leading to complicated adjustment of sensitivity.
Moreover, in the case where an object to be inspected is a semiconductor wafer, even if the quantity of displacement is correctly determined, a difference in film thickness in the wafer causes a difference in brightness between the same patterns of the image to be inspected and the reference image as indicated by reference character 4a in FIG. 4A and reference character 4b in FIG. 4B. A value of the difference in brightness becomes large as indicated by reference character 4c in FIG. 4C. This is also a false report. In order to prevent such a false report from being detected, as shown in FIG. 4D, a threshold value TH should be increased to a value like TH2. Otherwise, an individual threshold value should be given to each of an area having irregularity in brightness and an area not having irregularity in brightness.
In order to solve the problems of the conventional inspection technologies, the present invention provides a defect detection method and an apparatus therefor characterized by the following: in comparison inspection that compares an image to be inspected with a reference image to detect a defect judging from a difference between the images, decreasing the sensitivity only for a high-contrast part in a targeted image before inspection enables reduction in the number of false reports caused by misalignment as well as achievement of high sensitivity as a whole. In addition, the present invention provides a highly-sensitive comparison inspection method and an apparatus therefor, characterized by the following: a decrease in sensitivity of detection is kept within a required minimum by monitoring the accuracy of alignment so as to decrease the sensitivity of inspection only when misalignment occurs.
Moreover, in order to solve the problems of the conventional inspection technologies, the present invention provides a comparison inspection method and the apparatus therefor characterized by the following: in comparison inspection that compares an image to be inspected with a reference image to detect a defect judging from a difference between the images, adjusting the brightness at the edges of a high-contrast pattern in a target image before inspection so that a difference becomes small enables not only reduction in the number of false reports caused by an alignment error, but also achievement of high sensitivity, without increasing a threshold value TH. Furthermore, in the case of the inspection targeted for a semiconductor wafer, which produces the problem of irregularity in brightness of a pattern caused by a difference between film thicknesses, the present invention can reduce the number of false reports caused by the irregularity in brightness and realize inspection with high sensitivity without increasing the threshold value TH by adjusting the brightness of both images before the inspection.